1. Field of the Invention
The present invention relates to a heat exchanger unit for use in an automotive air conditioning system and, more particularly, to a heat exchanger unit having an improved external pipe connection.
2. Description of Related Art
A known heat exchanger unit 70 is depicted in FIG. 1. Heat exchanger 70 comprises a case 71 and a heat exchanger 72. Case 71 comprises an upper case member 73 and a lower case member 74 made of plastic or resin. Within case 71, heat exchanger 72 is accommodated and fixed. An inlet pipe 75 and an outlet pipe 76 for refrigerant are connected to heat exchanger 72. The open ends of both pipes are fixed to a flange member 77. Flange member 77 is sandwiched by receptor portion 78 provided on the lower periphery of upper case member 73 and receptor portion 79 provided on the upper periphery of the lower case member 74, and fixed therebetween. Flange member 77 is the external pipe connection portion that is the focus of the present invention.
In FIG. 2, heat exchanger 72 is depicted before being accommodated into case 71. Here, flange member 77, inlet pipe 75, and outlet pipe 76 have to be fixed to each other by pipe expanding, before heat exchanger 72 is accommodated into case 71. In FIGS. 3. through 5, the pipe expanding process is shown. FIG. 3 illustrates flange member 77 and outlet pipe 76 before pipe expanding. FIG. 4 illustrates the condition of outlet pipe 76 during pipe expanding, i.e., a core bar 100 is inserted into outlet pipe 76. FIG. 5 illustrates the condition of outlet pipe 76 after the pipe expanding. Generally, it is required to control the diameter of the external pipe connection portion with a high degree of precision in order to prevent refrigerant leakage. That is, with reference to FIG. 5, the precision of the diameter d' after the pipe expanding must be within a tolerance of plus minus 0.02 mm. This requirement of a high degree of precision of the diameter after pipe expanding necessitates a high degree of precision before pipe expanding too. In particular, with reference to FIG. 3, the diameter of an inner peripheral surface 77s of flange member 77 also must be within the tolerance limit of plus minus 0.02 mm. To establish and maintain such a relatively high degree of precision in shaping inner peripheral surface 77s of flange member 77, a conforming tool is required, which is expensive. In addition, because flange member 77 is formed of a relatively large block of aluminum, it requires a significant amount of material to manufacture. This factor also increases the manufacturing cost of the heat exchanger unit as a whole.
In the field of automotive air conditioning systems, there is a strong desire to keep cost down. As explained above, the external pipe connection part of a known heat exchanger structure, that is flange member 77 has been an obstacle to further cost reductions in manufacturing heat exchanger unit 70. Thus, for further cost reductions, it has been long desired to improve the structure of the external pipe connection portion of heat exchanger unit 70.